Involving Anion Interference

There are two viewpoints concerning the role of oxygenic anions in the kinetics of active dissolution of iron in acid media in the Tafel range of polarization. 

According to Russian researchers, there is a consecutive charge transfer via the intermediate Fe (OH)ad, the anions HSO4 and sol being reactant species taking part in the second step of the electron transfer, i.e.. 

This kind of mechanism accounts for the positive reaction order in SO found experimentally and exhibits the incompatibility between the low steady-state Tafel slope (30-40 mV dec ) and the inductive behavior of the electrode. 

On the contrary, according to the second viewpoint, near the corrosion potential oxygenic anions are slowly chemisorbed, more strongly at the dissolution sites than at the remaining atoms, thereby reducing the area free for metal dissolution and thus exerting an inhibitory effect. Thus, the 

In any event, any inhibiting effect must cease at the potential of unpolarizability, where the rate at which the adsorbed species leave the metal surface is higher than the rate of adsorption.  

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One of the most important factors involved in control of cation-anion interference effects is flame temperature. If compound formation occurs in the flame, as happens with calcium and the phosphate ion, a severe signal depression is observed. Use of high-temperature flames can minimize this effect. Flames that have been used successfully for this purpose are the nitrous oxide-acetylene flame and the premixed oxygen-acetylene flame. 

A method suitable for analysis of sulfur dioxide in ambient air and sensitive to 0.003—5 ppm involves aspirating a measured air sample through a solution of potassium or sodium tetrachloromercurate, with the resultant formation of a dichlorosulfitomercurate. Ethylenediaminetetraacetic acid (EDTA) disodium salt is added to this solution to complex heavy metals which can interfere by oxidation of the sulfur dioxide. The sample is also treated with 0.6 wt % sulfamic acid to destroy any nitrite anions. Then the sample is treated with formaldehyde and specially purified acid-bleached rosaniline containing phosphoric acid to control pH. This reacts with the dichlorosulfitomercurate to form an intensely colored rosaniline—methanesulfonic acid. The pH of the solution is adjusted to 1.6 0.1 with phosphoric acid, and the absorbance is read spectrophotometricaHy at 548 nm (273). 

Poloxamers are used primarily in aqueous solution and may be quantified in the aqueous phase by the use of compleximetric methods. However, a major limitation is that these techniques are essentially only capable of quantifying alkylene oxide groups and are by no means selective for poloxamers. The basis of these methods is the formation of a complex between a metal ion and the oxygen atoms that form the ether linkages. Reaction of this complex with an anion leads to the formation of a salt that, after precipitation or extraction, may be used for quantitation. A method reported to be rapid, simple, and consistently reproducible [18] involves a two-phase titration, which eliminates interferences from anionic surfactants. The poloxamer is complexed with potassium ions in an alkaline aqueous solution and extracted into dichloromethane as an ion pair with the titrant, tet-rakis (4-fluorophenyl) borate. The end point is defined by a color change resulting from the complexation of the indicator, Victoria Blue B, with excess titrant. The Wickbold [19] method, widely used to determine nonionic surfactants, has been applied to poloxamer type surfactants 120]. Essentially the method involves the formation in the presence of barium ions of a complex be-... 

Other useful solid-state electrodes are based on silver compounds (particularly silver sulfide). Silver sulfide is an ionic conductor, in which silver ions are the mobile ions. Mixed pellets containing Ag2S-AgX (where X = Cl, Br, I, SCN) have been successfiilly used for the determination of one of these particular anions. The behavior of these electrodes is determined primarily by the solubility products involved. The relative solubility products of various ions with Ag+ thus dictate the selectivity (i.e., kt] = KSp(Agf)/KSP(Aw)). Consequently, the iodide electrode (membrane of Ag2S/AgI) displays high selectivity over Br- and Cl-. In contrast, die chloride electrode suffers from severe interference from Br- and I-. Similarly, mixtures of silver sulfide with CdS, CuS, or PbS provide membranes that are responsive to Cd2+, Cu2+, or Pb2+, respectively. A limitation of these mixed-salt electrodes is tiiat the solubility of die second salt must be much larger than that of silver sulfide. A silver sulfide membrane by itself responds to either S2- or Ag+ ions, down to die 10-8M level. 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

Ozone might interfere with the intracellular bactericidal capabilities of alveolar macrophages by inactivating lysosomal hydrolases, or perhaps through the destruction of heme-containing enzymes that are apparentiy involved in producing superoxide anion radical. Further evaluation of the process by which relatively low concentrations of ozone potentiate bacterial infection would be of value. 

How then can we distinguish kinetically between these two possible mechanisms One possibility is to study the reactions under conditions where most of the substrate is in the free ion form. If the solvolytic reaction did not interfere, a first-order rate dependence on anion concentration would be observed if the reaction were bimolecular since only the ion pair can be involved in the substitution. 

Further, in atomic spectrometry we must face the serious problem that the behaviour (atomisation/excitation characteristics) of the analyte in the calibration samples should be the same as in the future unknown samples where the analyte of interest has to be quantified, otherwise peak displacement and changes of the peak shape may cause serious bias in the predictions. Fortunately, many atomic techniques analyse aqueous extracts or acid solutions of the (pretreated) samples and current working procedures match the amount of acids in the calibration and treated samples, so the matrices become rather similar. Current practices in method development involve studying potential interferents. The analyte is fixed at some average concentration (sometimes studies are made at different concentrations) and the effects of a wide number of potential interferents are tested. They include major cations, anions and... 

Colorimetric methods are most common and widely employed in environmental wet analysis. Most anions, all metals, and many physical and aggregate properties can be determined by colorimetric technique, which is fast and cost-effective. The method may, however, be unreliable for dirty and colored samples. Often, the presence of certain substances in samples can interfere with the test. In addition, if the color formation involves a weak color such as yellow, additional confirmatory tests should be performed. Despite these drawbacks, colorimetry is often the method of choice for a number of wet analyses. 

The following table lists the most commonly used solid membrane electrodes, their applications, and major interferences.1 Often the membrane is composed of a salt (listed first) and a matrix (listed second). Thus, a AgCl-Ag2S electrode involves the finely divided AgCl in a Ag2S matrix. The salt should be more soluble than the matrix, but insoluble enough so that its equilibrium solubility gives a lower anion (Cl-) activity than that of the sample solution. 

In an EC2j process, the initial ET step is followed by a second-order irreversible homogeneous reaction. For example, the feedback mode of SECM was employed to study the reductive hydrodimerization of the dimethyl fumarate (DF) radical anion [22]. The experiments were carried out in solutions containing either 5.15 or 11.5 mM DF and 0.1 M tetrabutylammonium tetrafluoroborate in A,A,-dimethyl form amide (DMF). The increase in the feedback current with increasing concentration of DF indicated that the homogeneous step involved in this process is not a first-order reaction. The analysis of the data based on the EC2 theory yielded the fc2 values of 180M-1 s-1 and 160M-1 s-1 for two different concentrations. Another second order reaction studied by the TG/SC mode was oxidative dimerization of 4-nitrophenolate (ArO-) in acetonitrile [23]. In this experiment, the tip was placed at a fixed distance from the substrate. The d value was determined from the positive feedback current of benzoquinone, which did not interfere with the reaction of interest. The dimerization rate constant of (1.2 0.3) x 108 M x s-1 was obtained for different concentrations of ArO-. 

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